This invention relates to a process and device for the manufacture of filamentary abrasive particles having controllable and preferably substantially constant aspect ratios.
Abrasive materials such as filamentary abrasive particles and grits are widely used in the production of abrasive devices such as grinding wheels, segments, belts, disks, polishing powders and the like. All of these materials require the use of very high temperatures in their manufacture. Sol-gel alpha alumina abrasive materials, in particular, have recently become more widely used in bonded abrasive products. Sol-gel aluminous abrasives have demonstrated advantages over other abrasive materials such as those described above. Such sol-gel abrasives are generally made by forming a hydrated alumina gel which may also contain varying amounts of additives such as MgO or ZrO.sub.2 and then drying and sintering the gel. See for example, U.S. Pat. No. 4,314,827.
Seeded sol-gel abrasives such as those formed by the conversion of hydrated alumina to alpha alumina using a seed material have also become very useful in making abrasive materials, particularly coated abrasives. See for example, U.S. Pat. No. 4,623,364.
Once a gel has been created, it may be shaped by any convenient method such as pressing, molding, or extruding and then carefully dried to produce an uncracked body of the desired shape. If abrasive material is desired, the dried extruded gel material is usually crushed or broken by suitable means such as a hammer or ball mill to form abrasive particles or grains. The same type of extrusion, grinding, and forming processes may be used with other abrasive forming compositions than sol-gel materials. See for example, U.S. Pat. No. 3,387,957 which discloses the production of elongated geometrical shaped and controlled grit size aluminous abrasive materials by means of extrusion.
The problem with the filamentary abrasive particles produced by the conventional extrusion, drying, and cutting techniques, be they prepared from a gel or from other abrasive materials, is that they usually have a wide distribution of aspect ratios. As used herein, "aspect ratio" refers to the ratio between the length along the principal or longer dimension and the greatest extent of the filamentary particle along any dimension perpendicular to the principle dimension. Where the cross-section is other than round, e.g. polygonal, the longest measurement perpendicular to the lengthwise direction is used in determining the aspect ratio. The filamentary particles in the distribution with extremely high aspect ratio values create problems, especially during coated abrasive manufacturing processes. Extremely long filamentary particles can result in shorting out of an electrostatic field which is often used during the coating process. Also, during sizing operations (in which a layer of adhesive is applied by a rotation roller on top of the filamentary particles) extremely high aspect ratio materials often are either pushed over by the roller or become embedded in the rubber of the rollers. In addition, the high aspect ratio materials often fracture, resulting in a build-up of grit on the roller and reduction in the actual aspect ratio of the grits in the product.